x=24
By using proportions, each side of a triangle is equal to that of another triangle on the same side. We see that the first triangle is 30/15 and the second is x/12. Since each sides are technically a scale factor of each other, hence the same shape, you would write an equation with an equal sign.
x/12=30/15 Multiply 12 to both sides
x=(30•12)/15 Simplify
x=360/15 Simplify
x=24
I hope this helped! If so, please mark brainliest!